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When a man walks outside barefoot, he unknowingly encounters tiny bugs on his feet. One or more parasites crawl on his skin and then through the skin. Once inside, these hookworms move until they enter their victim’s stomach. They attach themselves to their host’s stomach and feed on his blood from there.

This is not the plot of some horror movie. This is something that happens to one in four people in the world. That’s 1.9 billion people. But a new study in rodents shows that feeding these worms has benefits such as weight control and improved immunity.

Do not ignore the fact that hookworms can bring trouble. They feed on their host’s blood and are low in iron. This condition, called anemia (Uh-NEE-mee-uh), can make it difficult for the body to carry its normal load of oxygen through the bloodstream. Worms can also cause painful rashes and stunt growth in children. Few doctors prescribe worms to their patients. But new data show how mammals may have evolved to cope with some common and unpleasant infections. And people have been fighting hookworms for a long time. Even ancient mummies show signs of infection.

Over the past century, hookworm infections have become rare in developed countries such as the United States.

In less industrialized countries, “infectious diseases, including worm infections, are common,” noted Haining Shi. He studies pediatrics at Harvard Medical School in Boston, Mass. People living in such countries have lower rates of chronic (long-term) disease than those in industrialized countries, he added. This prompted Shi’s team to test an idea: could parasites help prevent some serious, chronic diseases?

The group decided to focus on obesity. One-third of US adults are obese. One in six children is overweight or obese. Carrying too much body fat can lead to other serious health problems, including heart disease and type 2 diabetes.

Obesity is caused by consuming more energy and food than the body can expend. But not all bodies use energy with the same efficiency. Shi’s team shows that the parasite changes how efficiently the body uses energy — at least in mice.

How they found it

The researchers chose mice as a model for what might happen to humans. Like humans, mice fed a high-fat diet tend to become obese. These animals can also harbor parasites. The parasite Shea chose to work with, Heligmosomoides polygyrus (Heh-LIG-moh-soh-MOY-ih-dees Pah-lee-GY-rus), is not a hookworm. And it does not live in people. Hookworms, on the other hand, are nematodes (NEE-muh-toad). And this worm is a hookworm-like nematode that infects humans.

The scientists divided the mice into four groups. Both groups were fed a normal diet. The rest ate high-fat meals. The researchers infected a group of mice with nematode larvae on two diets. Shi’s group then measured the growth of the animals up to 105 days.

Uninfected mice fed a high-fat diet almost doubled in size. But they were the only group to do so. All animals fed the regular diet grew normally. The same was true for worm-infected mice fed a high-fat diet.

If the infected mice lost weight due to bleeding from the worms, why didn’t those on a normal diet lose weight compared to uninfected mice? The answer was a bit more complicated, Shi’s group showed. The worms changed how the infected mice processed the high-fat diet.

When uninfected mice became obese, their blood sugar levels increased. Fats like cholesterol (Koh-LES-tur-awl) and triglycerides (Try-GLIH-sur-eydz) also increased in the blood. Even his liver was fatty. And that was not all. Different fat-related genes are activated in these animals.

Mice fed a normal diet showed no such changes. And mice with worms did not eat a high-fat diet.

Worms even influenced what type of fat they stored. Like humans, there are two types of mice. White fat is an energy-dense type that can be burned at some point in the future. Think of it as stored fuel in the body. Brown fat is enriched with special molecules called break down proteins. This protein activates the body’s energy burning mechanism.

Obese mice usually have white fat. Worm-fed rodents fed a high-fat diet were rewarded with brown fat.

Indeed, Shi now reports that “worms have been found to have beneficial effects, particularly in mice fed a high-fat diet.” His team announced the results in March

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